psi scientific report 2008 · 2020-01-09 · the year 2008 marked the 20th anniversary of the paul...

PSI Scientific Report 2008

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Cover photo:

Control room of the Low Emittance

Gun test stand, where critical

components for XFEL‘s electron

source are being tested.


Published byPaul Scherrer Institute

EditorPaul Piwnicki

English language editingTrevor Dury

CoordinationEvelyne Gisler

Design and LayoutMonika Blétry

Photographs© Paul Scherrer Institute

Printing Ostschweiz Druck AG, Wittenbach

Available fromPaul Scherrer InstituteCommunications Services5232 Villigen PSI, SwitzerlandPhone +41 (0)56 310 21 11www.psi.ch

PSI public [email protected]

Communications officerDagmar Baroke

ISSN 1662-1719

Copying is welcomed, provided the source is acknowledged and an archive copy sent to PSI.

Paul Scherrer Institute, April 2009

4 Building on our past to prepare our future

Foreword from the director

7 PSI-XFEL

17 Research focus and highlights

18 Synchrotron light

28 Neutrons and muons

36 Particle physics and nuclear chemistry

42 Micro- and nanotechnology

46 Biomolecular research

50 Radiopharmacy

54 Large research facilities

56 Proton therapy

60 General energy

70 CCEM-CH

72 Nuclear energy and safety

84 Environment and energy systems analysis

91 User facilities

92 PSI accelerators

96 Swiss Light Source SLS

98 Spallation Neutron Source SINQ

100 Swiss Muon Source SµS

101 Ultra-Cold Neutron Source

102 Tandem accelerator

105 Technology transfer

113 Facts and figures

114 The year 2008 in numbers

116 Commission and committees

119 Publications

Table of contents 3

The year 2008 marked the 20th anniversary of the Paul Scher-

rer Institute, PSI, and my colleagues seized the opportunity

to organise and run several special events during the year,

with the ultimate goal of giving the Institute a higher visibil-

ity in the neighbourhood, among critical non-scientific stake-

holders and within Switzerland in general. At the same time,

important scientific and technological results have been ob-

tained, of which you will learn more in this report. Finally, 2008

was also a special year for me, as I was honoured with the

Directorship of the Institute.

20 years Paul Scherrer Institute

In 1988, PSI was founded by the merger of the Swiss Institute

for Nuclear Research and the Federal Institute for Reactor

Research. The cultures of both institutes were very different

at that time, making a new, joint beginning quite difficult.

However, from today’s point of view, the amalgamation was

the right decision: With the focus on the research areas of

solid-state research and materials sciences, particle physics,

life sciences, energy research and environmental research,

a sagacious decision can be judged to have been made.

Nowadays, PSI’s concept of focusing on its large-scale facilities

– the neutron and muon sources around the proton accelera-

tor and the Swiss Light Source SLS – is considered a success.

The Institute focuses, on the one hand, on providing service

for external research groups, which receive the support they

need as they use the facilities, beamlines and research instru-

ments, whereby it is our strategy to excel in a number of se-

lected disciplines, rather than trying to serve the needs of all

users. On the other hand, PSI’s own research concentrates on

those research topics where an advantage in terms of inter-

national competition can be gained by employing our own

in-house large-scale and complex research equipments.

In addition, PSI’s own research on the complex research equip-

ment itself results in the acquisition of experience that can be

used to develop our facilities still further, maintaining the

latter’s ability to compete internationally.

Three requirements that are essential for success

PSI serves as a successful example of how a research insti -

tute can continue to be an internationally acknowledged

scientific hub by simply remaining flexible and thus safe-

guarding its own existence. Three prerequisites are essential

for this:

Firstly, a well-defined scientific goal and a clear understanding

of the Institute’s role in the Swiss research landscape, espe-

cially its relationship with the universities; secondly, political

decision-makers who understand the importance of basic

and applied research for the progress of society, and conse-

quently support us; thirdly, excellent staff. Only with highly

qualified, experienced and motivated personnel is success in

performing cutting-edge research possible.

Based on these three factors, within the course of the last

20 years PSI has been able on the one hand to generate out-

standing fundamental research results and on the other hand

to develop key technologies and introduce them successfully

to the market. To give you two examples:

Firstly, the development of compact accelerators for the pro-

ton therapy of tumours. PSI is a technology leader in this area,

and recent developments can be seen on page 56. Several

Building on our past to prepare our future

hospitals have already expressed their intention to establish

this technology on their own sites.

And secondly, we have developed detectors that are orders

of magnitude more sensitive than those existing previously.

One such example is the MYTHEN X-ray detector, which is

presented on page 26. In combination with recent develop-

ments at the SLS, MYTHEN is opening up wholly new perspec-

tives for diffraction experiments.

Both products have already been successfully introduced to

the market. It should, however, not go unmentioned that both

technologies are the belated offspring of the basic research

undertaken in the field of particle physics. As such, they are

the results of a development phase of more than 20 years.

Where else would such a long-term endeavour be possible, if

not at a publicly funded research institute?

Interesting and surprising findings

As to our scientific achievements in 2008, let me just highlight

a couple, details of which you will find in the individual chap-

ters in this report: Interesting and even surprising findings

around superconductivity and magnetism revealed using

neutron scattering and muon spin resonance accompanied us

throughout the year (p.28–31); using the high spatial resolu-

tion of synchrotron light at the SLS it was possible on the one

hand to create new nano-structures (p.42–45) and on the

other hand to reveal microscopic details of the functioning of

photo-catalysts (p.20), fuel cells (p.68) and bio-molecules

(p.23) with unprecedented accuracy. To complement the work

performed at our large-scale facilities, various complemen-

tary methods are currently developed in Biology, Energy, or

Environmental Sciences. For example, by using selected iso-

topes it is now possible to date glacier ice with unequalled

precision (p.40), to enhance the NMR sensitivity for potential

medical diagnosis (p.32), to develop efficient SPECT tracers

(p.50), or to assess the long-term safety of radioactive waste

repositories (p.82). On the operational side of the PSI accel-

erators, two world records were achieved: The proton facility

surpassed its own world record, with a new beam power of

1.3 MW, and the SLS operating team announced a significant

improvement of beam quality, resulting in a world-record low

vertical emittance of 2.5 pm rad.

For the time being, PSI fulfils all the criteria necessary for

remaining amongst the world’s top research institutes for the

next 20 years. For us, one such criterion is the development

and construction of a novel and ambitious large-scale research

installation for dynamical studies with femtosecond and

atomic resolution: the free electron laser PSI-XFEL, whose

commissioning is planned for 2016 (p. 7).

As a good and longstanding tradition, I shall end this foreword

with my sincere thanks: Thanks to the PSI staff, who have

made everything possible on which we proudly report in this

volume, and “Thank you” to our research and development

partners in academia and industry worldwide, to our home

canton of Aargau for its manifold support, and to the Board

of the ETH and the Swiss Federal Government for their con-

tinued support.

Joël Mesot, Director

Foreword 5

“One of the key ingredients

in the success story of PSI

is the quality of its staff”

The PSI-XFEL is planned to be the next large-scale facility at the Paul

Scherrer Institute and will contribute to the vitality of the laboratory

during the coming decades. The project represents a continuation of

PSI’s excellence in the field of synchrotron radiation research, estab-

lished through the outstanding performance of the Swiss Light Source

(SLS), which began operation in 2001.

The PSI-XFEL will complement the SLS by being ideally suited for

experiments where the combination of atomic spatial resolution and

femtosecond temporal resolution is required – detailed images of

atoms and molecules in motion will be captured for the first time.

The PSI-XFEL will be one of the first national free-electron laser

facilities worldwide that aims to produce coherent light with wave-

lengths down to 1 Ångström. It will hopefully serve as a model for

other national sources, since further projects of this type are a long-

term necessity, given the limited number of experiments that can be

installed at any one such facility.

With the PSI-XFEL, Swiss and external users will have an excellent

scientific instrument with which to perform novel investigations in

the fields of chemistry, biochemistry, condensed matter physics and

materials science.

New concepts and innovative technical solutions have been incorpo-

rated into the facility design to optimize performance and minimize

cost. The low-charge concept, combined with an ultra-small electron-

beam emittance, is the essence of this design. The higher longitudi-

nal pulse compression required is realized with a newly-developed

dual-frequency accelerating cavity. High-gradient and high-voltage

acceleration systems are being developed to reduce space charge

effects and to guarantee the required electron beam characteristics

for the lasing process.

PSI-XFEL

8 XFEL – Project overview and new developments

7

Romain Ganter, scientist at the PSI-XFEL

project, adjusting the intensity of

the laser beam which will generate the

electron beam in XFEL’s electron gun.

The PSI X-ray Free Electron Laser – XFEL

Hans Braun, Romain Ganter, Marco Pedrozzi, Sven Reiche, Albin F. Wrulich, Free Electron Laser Project

(PSI-XFEL), PSI; Leonid Rivkin, Department Large Research Facilities, PSI

The exciting features of this novel light source will, for example, allow users to unravel the molecular structure

of a protein and to effectively take a motion picture of a chemical process on the scale of femtoseconds (fs).

X-ray light of unprecedented quality is needed to guarantee the accomplishment of these ambitious goals. This,

in turn, requires an electron beam with high performance and sophisticated beam-handling. In the past year,

important steps towards the technical realization of the facility were made, and the XFEL concept was further

improved.

Project overview

In a Free Electron Laser (FEL), electrons are not bound to an

atom, as in a conventional laser, and light is created by trans-

verse acceleration of a relativistic electron beam in an undu-

lator. In a conventional laser, coherence is created by a

stimulated transition of the electrons from an excited state of

the atom to the ground state, with a corresponding emission

of light that forms a narrow bandwidth around a single wave-

length (the shortest wavelength possible is in the VUV). In a

FEL, coherence arises from the interaction of the emitted

electromagnetic wave with the electron beam, and lasing

wavelengths can be achieved continuously down to the hard

X-ray regime.

The generic elements of a FEL are a linear accelerator, a ra-

diator constructed from several undulators, with beam focus-

ing devices positioned between the undulator sections, and

the photon beam distribution lines that house the experiments

at their ends.

Acceleration to high energies is necessary for two reasons.

Firstly, the resonance wavelength of an undulator for a given

(minimum feasible) period length is reduced with the square

of the energy, i.e. short wavelengths require higher energies.

Secondly, the electrons can only emit in the fundamental ra-

diation mode if the beam size and divergence (expressed by

their product, the emittance) are small. Fortunately, the trans-

verse beam size (and emittance) of the electron beam in a

linear accelerator decreases with increasing energy (adia batic

damping). However, the latter condition requires high electron

energies (and costly, long linear accelerators) for short lasing

wavelengths.

In addition to the requirement of a small electron beam cross-

section, there is also the pre-condition that many particles

are to be involved in the process, i.e. the charge density must

be high. This is achieved by compressing the length of the

electron bunch in the linear accelerator by a sequence of bunch

compressors.

In the PSI-XFEL, the acceleration process starts at the cathode

of the electron gun. Two different electron guns are foreseen

for the three undulator lines (Figure 1, Athos: 7 nm – 3 nm;

Porthos: 3 nm – 0.7 nm; Aramis: 0.7 nm – 0.1 nm). Since the

quality requirements are less stringent for the longer wave-

lengths, a more conventional gun, based on photoemission,

can be used here. For the baseline design incorporating the

CERN CTF3 gun, an electron pulse (bunch) of 10 ps duration

(fwhm) and a peak current of 22 A is extracted from a metallic

Figure 1: Conceptual layout of the undulator lines.

8 PSI-XFEL PSI Scientific Report 2008

or semiconductor surface by means of a laser beam. The

cathode is placed on the axis of a 2½ cell, 3 GHz accelerating cavity, which immediately accelerates the electron bunch after

extraction from the cathode. Solenoid and quadrupole mag-

nets in the subsequent structure focus the beam, to minimize

the emittance at the exit of the gun complex.

For the second gun, several options are possible. The decision

on which will be based on the success of ongoing R&D work.

It will either be a newly-developed photo-electron gun or an

alternative gun based on field emission arrays, where electrons

are extracted from a surface by means of high electric field

gradients (~ 5 GV/m). Such high gradients can be easily

achieved if the field is applied to micro- or nano-structured

surfaces where the field is strongly enhanced around tips with

small apex radii. In order to mitigate space charge effects, the

energy of the beam is rapidly increased by passing the beam

through a high-voltage and high-gradient diode configuration,

before entering the first RF accelerating structure. A newly-

developed high voltage pulser is currently being tested and

further developed. Different surface materials are being

explored, to discover those which can sustain high surface

gradients without breakdown. Since this concept relies on a

longer initial pulse (40 ps fwhm with 5.5 A peak current), a

higher compression is required to reach a sufficiently high

peak current at the entrance to the undulator. This compres-

sion starts in the first accelerating cavity, which is fed by two

frequencies (1.5 GHz and 4.5 GHz). In this way, the longitudi-

nal energy distribution in the beam can be suitably shaped to

reach a very effective velocity compression. In the low relativ-

istic regime, particles with different energies still have a

notable difference in velocities. If they are arranged properly

in energy along the bunch, they move towards the bunch

centre, and the length is reduced.

After the gun complex, the bunch can be directed into a diag-

nostic line for complete characterization. A more conven-

tional accelerating structure follows the gun and comprises

four S-band structures of 4 m length, surrounded by focusing

solenoids. The maximum accelerating gradient is 20 MV/m.

In the test setup for this injector presently under construction,

a bunch compressor will be placed at the end (250 MeV) for

test purposes. In the final layout, an additional accelerating

section will be added (Linac 1) in front of the bunch compres-

sor, boosting the energy to 450 MeV. The higher energy will

alleviate the risk of emittance dilution due to space charge

effects in the bunch compressor. Linac 1 comprises two FODO

cells, each of 10 m length, with two accelerating structures of

2 m length between adjacent quadrupoles. One cell will pro-

vide an energy increase of 120 MeV on crest, corresponding

to an accelerating gradient of 30 MV/m.

During the acceleration process prior to the bunch compressor,

an energy chirp will be introduced in the beam. Particles with

higher energies will be arranged at the tail of the bunch and

particles with lower energies at the head of the bunch. Due

to the nonlinearity of the 3 GHz accelerating field, the energy

chirp is slightly too large in the head of the bunch and too

small in the tail. Therefore an X-band (12 GHz) cavity is intro-

duced before the bunch compressor to compensate for these

deviations.

The bunch compressor (BC1) consists of a sequence of four

bending magnets, which create an orbit bump around the

straight motion path in the linac. Since particles with higher

energies are subject to a smaller deflection in the magnets,

their orbit lengths are shortened. They are consequently

moved from the tail towards the centre of the bunch. Simi-

larly, the lower-energy particles at the head of the bunch ex-

perience larger deflections that result in a lengthening of the

orbit and a transition towards the bunch centre. The net effect

after BC1 is that the length of the bunch is reduced from 10 ps

(for the 200 pC mode) to 450 fs.

The subsequent Linac 2 (with the same cell structure as

Linac 1) raises the energy to 2.1 GeV. At this point, the second

magnetic bunch compressor (BC2) is introduced, which re-

duces the bunch duration to 30 fs, with a corresponding in-

crease of the peak current to 2.7 kA. For the succeeding Linac 3,

the transverse beam dimensions are already considerably

smaller, due to the increased beam energy, permitting the

distance between the focusing quadrupoles to be increased.

One cell here is constructed from four two-metre-long accel-

erating sections between two adjacent quadrupoles, and has

a total length of 19 m.

After Linac 3, the electron beam is extracted for the longer-

wavelength FEL lines Athos and Porthos. The nominal energy

at this point is 3.4 GeV, but will be reduced to 2.1 GeV for

Athos by not powering Linac 3. It remains to be verified by

simulations whether the focusing lattice can remain un-

changed, since the quadrupole strengths are matched to a

higher energy, otherwise a second extraction point after Linac 2

will need to be inserted.

Only for the 1 Ångstrom wavelength of Aramis is an addi-

tional boost to 5.8 GeV required, provided by Linac 4, which

uses the same cell structure as Linac 3.

The electron beam quality is now sufficient for the lasing

process as the beam enters the undulators. The emittance is

reduced by adiabatic damping, and the bunch is longitudi-

nally compressed.

In principle, an electron transversally accelerated in a mag-

netic field emits a broad spectrum of radiation. However, in

an undulator the only wavelengths not to be eliminated by

interference effects are those for which the electron beam lags

behind the photon beam by one wavelength (or an odd integer

multiple). Due to the long undulator structure, the intensity

of the radiation steadily increases and becomes sufficiently

PSI Scientific Report 2008 PSI-XFEL 9

strong to act back on the electron bunch. The transverse

electric field of the emitted wave causes acceleration and

deceleration of particles within the transversally moving

electron bunch in the undulator, which imprints a micro-bunch

structure onto the whole. The more this structure is enhanced,

the more coherent the radiation becomes. At saturation, the

waves emitted from the different micro-bunches are summed

up in phase, leading to a tremendous increase in intensity of

the transversally, fully coherent light.

At the end of the undulator, a photon beam with 2.9 GW

power is extracted from Aramis, with a pulse duration of

40 fs at 1 Ångström wavelength.

The photon beam is then distributed to the various experi-

ments. At the exit of the undulator, no material can withstand

the high power density, necessitating long expansion lines

before optical elements can be positioned in regions of ac-

ceptable heat load. Since X-ray mirrors have useful reflectiv-

ity only at very small grazing angles, long optical lines with

refocusing are required to guide the photon beam to the ex-

periments.

Project progress

The PSI-XFEL project is being executed in three parallel devel-

opments. Major emphasis is given to the realization of a low-

emittance gun by exploring the ultimate limits of conven-

tional photo-cathodes and investigating new options based

on field-emission from needles and field-emitter arrays (FEA).

Simultaneously, the injector of the XFEL facility is being built,

which will integrate the major critical R&D elements of the

project and allow their verification and optimization at an

early stage. Finally, the configuration of the final XFEL facility

is being developed and the civil engineering requirements are

being specified.

High-brightness electron beams

Operation of the PSI-XFEL will start with a conventional

photo-gun for the electron source. Simulations have confirmed

satisfactory performance for both the hard and soft X-ray

undulator beamlines. Eventually, after successful completion

of the R&D, the driver system for the hard X-ray line will be

equipped with a cathode based on field-emission from a

needle or an FEA, embedded in a diode configuration for high-

gradient and high-voltage acceleration.

For the needle cathode, two independent emittance measure-

ment methods have confirmed the target value of 0.2 µm.

Further work is needed to reach the required charge and emis-

sion current. A major step forward was made for FEAs by

controlling the tip apex for homogeneous emission, and a

production process for double-gated arrays (Figure 2) has

been developed [1]. It could be demonstrated that the focus-

ing gate has little effect on the emitted current, compared to

the single-gated array. So far, the current is limited by the

available accelerating voltage. A new test setup is being in-

stalled to overcome this limitation.

XFEL injector

Construction of the 250 MeV injector for the FEL facility will

allow the testing of critical technical developments, and the

verification and optimization of their performance, at an

early stage. For optimum performance, two complementary

electron guns will feed the linear accelerator. Both gun con-

cepts can be tested in the 250 MeV injector facility. Operation

will start with the “CTF” photo gun (Figure 3) [2]. Emission

from the cathode is driven by a Ti-Sapphire laser system, which

allows longitudinal pulse-shaping and wavelength-tuning for

the generation of minimum emittance.

Figure 3: Injector configuration with the CTF gun.

Figure 2: Double-gate

field emitter.

1 μm

Figure 4: Injector building.


Construction of the 250 MeV injector is currently in progress

and the procurement of magnets, accelerating structures,

klystrons, modulators and laser systems has begun. Building

construction is well underway (Figure 4) and will be com-

pleted early in 2010.

XFEL facility

Extensive start-to-end simulations have been performed in

order to consolidate the basic parameters and the configura-

tion of the XFEL facility. Figure 5 shows the simulation results

for Self Amplified Spontaneous Emission (SASE) at 1 Ångström

wavelength.

The three XFEL beamlines have been re-optimized to allow

independent operation. For the soft X-ray undulator line,

seeded operation is foreseen, possibly based on high-

harmonic generation from a Ti-Sapphire laser [3]. This will

enhance the longitudinal coherence of the XFEL pulse, even

at wavelengths down to 1 nm, and render the XFEL operation

more stable in both frequency and time. Provisions for short-

pulse operation have been made, based on either laser-slicing

or low-charge, “single-spike” operation (Figure 6).

The consolidation of the XFEL configuration has allowed the

preparation of a conceptual design of the building with ex-

perimental hall and technical infrastructure. The orientation

of the building has been slightly modified to increase the

available space (Figure 7).

The accelerator and the experimental hall will be completely

below ground, with an underground supply area on top of the

accelerator tunnel (Figure 8).

For further information see: http://fel.web.psi.ch

References

[1] S. Tsujino et al., to be published.

[2] R. Bossart, M. Dehler, Design of an RF-Gun for heavy

beam loading, Proc. EPAC 96 (1996).

[3] S. Reiche, PSI-XFEL Internal Report RS06–004 (2009).

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Figure 5: Spectrum at saturation for SASE operation.

Figure 6: “Single-spike” spectrum at saturation for 2 pC operation.

Figure 8: Design study of the

XFEL tunnel, with accelerator

and technical gallery.

Figure 7: Layout of the facility adjacent to the PSI site.

Injector

undulators Ex. hall545 m

930 m


Scientific strengths of the PSI-XFEL

The photon energies of the PSI X-ray Free Electron Laser (XFEL)

[1] will allow a wide range of investigations of matter at the

molecular and atomic level (see Figure 1). Furthermore, the

extremely short X-ray pulses (

will be possible to take “snapshots” of the instantaneous

magnetization distribution in thin-film nanostructures, and

hence to follow this process in detail.

Unstable intermediates in surface catalysis

Surface catalytic reactions play a central role in many indus-

trial chemical processes, in clean energy production and in

eliminating environmental pollutants. A typical reaction is

shown schematically in Figure 3. In the presence of a heated

substrate, reactant species go through a series of short-lived

intermediate states, finally emerging as the desired product.

Figure 3 illustrates a possible “THz pump / X-ray absorption

spectroscopy probe” XFEL measurement, which will elucidate

the chemical nature of intermediate states on a ps-ns time-

scale [3].

Protein structure from 2D-crystals

Protein structure determines the function of the building

blocks of life, and its knowledge permits the intelligent design

of drugs to treat genetic diseases. Many clinically relevant

proteins are membrane bound. Their 3D crystallization is dif-

ficult and requires tedious optimization to yield well-diffract-

ing crystals. With the PSI-XFEL, it should be possible to extract

high-resolution structural data from diffraction experiments

on two-dimensional crystals (see Figure 4), complementing

the techniques of electron diffraction/microscopy [4]. Although

each XFEL shot will locally destroy the sample, with a focus

spot size of 100 nm and the 100 Hz repetition rate of the PSI-

XFEL, it will be possible to reposition the sample between

shots.

References

[1] http://fel.web.psi.ch

[2] R. Hertel, S. Gliga, M. Fähnle, C.M. Schneider, Phys.

Rev. Lett. 98 117201 (2007); J. Raabe et al., ibid 94

217204 (2005).

[3] H. Ogasawara, et al., Proc. 27th FEL Conf. (2005);

A. Wokaun and I. Czekaj, private communication.

[4] M. van Heel et al., Quarterly Rev. Biophys. 33

307 (2000).

[5] C. Kewish, P. Thibault, O. Bunk, F. Pfeiffer, submitted

(2009).

Figure 2: Predicted magnetic behaviour [2] at the centre of a

Co-nanodisk, illustrating how a magnetic field pulse can change

the direction of a vortex core from up (red) to down (green).

Figure 4: A high-intensity XFEL pulse scatters on a 2D-membrane

protein crystal. Sufficient scattered photons are collected to

allow a structural solution before the pulse locally destroys the

sample [5].

Figure 3: The “Haber-Bosch” catalytic process for synthesizing

ammonia. At the PSI-XFEL, such a reaction may be initiated

with a THz pulse and probed at time τ later with soft X-ray spectroscopy [3].


Introduction

Operating an X-ray free electron laser at relatively low electron

energy requires an electron beam of unprecedented bright-

ness: the electrons must be as densely packed as possible

yet still propagate on highly parallel trajectories. Since any

irregularities in the electron beam from the source cannot be

corrected further downstream, the quality of the electron

source is of paramount importance. To explore and evaluate

new concepts for the generation of ultra-bright electron beams,

such as field-emitter arrays or needle cathodes, PSI initiated

the Low Emittance Gun (LEG) project. (The emittance of a beam

is a measure for how well it can be focused – the lower the

emittance, the brighter the beam.) The centre-piece of this

effort is the PSI-LEG test stand, located in the OBLA building.

The installation was implemented in two phases, with elec-

trons reaching energies of 500 keV and 4 MeV, respectively.

Phase I: From 0 to 500 keV in 50 ps

In its initial form, the PSI-LEG test stand consisted of a high-

voltage pulser followed by a short diagnostic beamline. In this

configuration, the test stand was in operation from December

2007 until October 2008.

The pulser generates a “diode” electric field between two

metal electrodes (typically copper or stainless steel) sepa-

rated by a variable gap of several millimetres. The electric field

can reach up to 120 MV/m for the duration of about 250 ns.

It accelerates electrons emitted at the cathode to a kinetic

energy of approximately 500 keV in a few tens of picoseconds.

At this energy, the influence of repulsive space-charge forces

The PSI-LEG test stand is PSI’s test bed for the development of an ultra-bright electron gun based on a high-

voltage pulser configuration. This is one of several promising candidate designs for the electron source to be

used at the PSI X-ray Free-Electron Laser. Since the start of operation at the end of 2007, the test stand has

provided important information on relevant materials and geometries. The facility was recently upgraded with

the addition of a radio-frequency cavity to accelerate electrons up to 4 MeV.

T. Schietinger, B. Beutner, R. Ganter, C. Gough, C. P. Hauri, R. Ischebeck, S. Ivkovic, Y. Kim, F. Le Pimpec,

K. B. Li, P. Ming, A. Oppelt, M. Paraliev, M. Pedrozzi, V. Schlott, B. Steffen, A. F. Wrulich, Free Electron Laser

Project (PSI-XFEL), PSI

The PSI-LEG test stand

Figure 1: The tungsten mask (“pepper-pot”) intercepting the

electron beam...

Figure 2: ...and the resulting electron density as imaged by

a screen further downstream. The fuzziness of the image is a

measure of the beam emittance.


is considerably reduced. The emission of electrons from the

metal cathode is either triggered by the electric field itself

(field emission), or by a UV laser beam shining onto the cath-

ode for a few picoseconds (photo-emission). The fraction of

photons that produce free electrons is called the quantum

efficiency, an important characteristic of the cathode mate-

rial. The laser beam enters the diode through a small hole in

the anode (the iris). Through the same iris, the accelerated

electrons are allowed to leave the diode gap and enter the

diagnostic beamline. A series of solenoid magnets and screens

then allows detailed characterization of the electron beam.

The emittance of the beam is determined either from the

observation of the beam size under progressively stronger

solenoid focusing or, more precisely, from the overall beam

size and the local uncorrelated divergence. The latter is esti-

mated with the so-called “pepper-pot” method, in which the

beam is sent through a tungsten mask pierced with an array

of small holes, much like the top of a pepper shaker (Figure1).

The broadening of the electron distribution emerging from

each hole is a direct measure of the local, uncorrelated diver-

gence of the beam (Figure 2).

Operation of the test stand during Phase I resulted in a wealth

of information important to the further development of the

programme. In particular, a wide range of cathode materials

was investigated with regard to quantum efficiency and high-

est field gradient achievable with and without laser irradiation.

Electrodes made from diamond-like carbon were shown to

withstand up to 240 MV/m without, and 100 MV/m with, laser

irradiation. The maximum extracted charge was 200 pC, when

using a powerful Nd:YAG laser of 262 nm wavelength. The

setup also allowed an accurate measurement of the so-called

thermal emittance of the electron beam emerging from a

metal cathode. This is the residual emittance arising from the

thermal motion of the electrons inside the cathode prior to

emission.

Phase II: Surfing to 4 MeV

To increase the beam energy into the MeV range, a radio-

frequency cavity was added to the test stand during a major

upgrade (Figure 4). The beamline now measures some five

metres in length and includes a dispersive branch for momen-

tum measurements (Figure 3). Installation was completed in

December 2008, and first beam was observed in early January

2009. The new setup will give valuable insights as to how the

emittance of the generated electrons can be preserved up to

higher energy.

An entirely re-designed laser system will provide laser pulses

of tuneable wavelength, thus allowing the study of beam

emittance as a function of photon energy. Last but not least,

the experience gained by operating the PSI-LEG will be of great

value for the commissioning of the much larger future facilities

that are planned in the context of the PSI-XFEL project.

Figure 4: Close-up of the cathode (magenta), anode (yellow) and

two-cell cavity (silver).

Figure 5: Proud members of the PSI-LEG team posing in front

of the beamline shortly before its upgrade to the 4 MeV

configuration.

Figure 3: Schematic layout of the PSI-LEG test stand with the full

4 MeV beamline.


Examples from PSI’s research portfolio in 2008 are presented on the

following pages, but this is only a very small sample of the cutting-

edge research being performed at the Institute.

A large number of results in various fields of science have been obtained

at PSI’s large-scale facilities; for example, research at SLS provided

insights into the structures of novel nanomaterials, the inner workings

of photocatalysts and processes in biomolecules. The fascinating in-

teractions between superconductivity and magnetism were among the

topics investigated with muons and neutrons.

The development of a new process for turning wet biomass into meth-

ane, and thus making the solar energy stored in these materials avail-

able for use in households and vehicles, is but one example of PSI’s

activities towards a sustainable energy supply. In the field of nuclear

energy and safety, current research projects include the investigation

of the geological conditions required for the storage of nuclear waste

and the development of methods for monitoring material fatigue in

nuclear power plants.

In environmental research, information gained from an ice core drilled

in the Siberian Altai Mountains showed the influence of solar activity

and greenhouse gases on the local climate, and a new method devel-

oped by researchers from PSI and ETHZ will allow even more precise

dating of ice cores in the future.

Activities in the medical field covered a very broad range, from fun-

damental research into the origins of various diseases to the treatment

of actual patients at the proton therapy facility. The year 2008 was

the first year of continuous patient treatment at Gantry 1, as well as

a year of considerable progress in the development of future facilities

and technologies for proton therapy at PSI.

Research focus and highlights

18 Synchrotron light

28 Neutrons and muons

36 Particle physics and nuclear chemistry

42 Micro- and nanotechnology

46 Biomolecular research

50 Radiopharmacy

54 Large research facilities

56 Proton therapy

60 General energy

70 CCEM-CH

72 Nuclear energy and safety

86 Environment and energy systems analysis

17

Reto Flückiger, PhD student in the

Electrochemistry Laboratory,

preparing a tomography experiment on

gas diffusion layers for fuel cells.

Structure and trapping properties of corrugated monolayers – new results from across the SLS

Domenico Martoccia, Matts Björck, Christian Schlepütz, Stephan Pauli, Bruce Patterson, Philip Willmott,

Hugo Dil, Luc Patthey, Swiss Light Source (SLS), PSI; Jorge Lobo-Checa, Nanolab, University of Basel;

Simon Berner, Thomas Brugger, Jürg Osterwalder, Thomas Greber, Physics Institute, University of Zurich

The physical properties of the isoelectronic, two-dimensional structures of graphene and hexagonal boron-nitride

are complementary and may also in combination become technologically useful. On solid supports, both devi-

ate from a perfectly flat honeycomb structure and provide the possibility to functionalize them as templates for

nanoscaled arrays among other applications. Structural and electronic studies of these systems performed at

the Swiss Light Source have provided new insights for their potential use in areas as diverse as molecular

recognition, nanoarrays, and novel electronic device fabrication.

Graphene and hexagonal boron-nitride (h-BN) are honeycomb

structures that can be grown as single layers, or “sheets”, on

crystalline substrates. The bonding between these sp2-hybrid-

ised, two-dimensional structures and the substrate varies

periodically, due to a moiré-like interference caused by differ-

ences in their respective in-plane lattice constants. As a

consequence, the atomic sheets become corrugated, resulting

in features with periods of a few tens of Ångströms. They are

characterised by pronounced and separated triangular eleva-

tions on a hexagonal network in the case of graphene, but in

h-BN the elevations are more hexagonal with wire-like con-

nected rings, and is thus referred to as a “nanomesh”. Their

future use as nanotemplates for molecular arrays and in

recognition of macromolecules is a tantalizing prospect that

can be better assessed only by a deeper understanding of

their structures and electronic properties. With this in mind,

studies of these systems have been performed at the Surface

Diffraction Station and Surface and Interface Spectroscopy

Beamline of the Swiss Light Source.

Graphene on Ruthenium

Initial studies of graphene on Ru(0001) (g/Ru) using tech-

niques such as scanning tunneling microscopy and low-ener-

gy electron-diffraction produced mutually contradictory re-

sults: two different structures were proposed – one in which

(1212) graphene hexagons lie on (1111) Ru unit cells

(denoted henceforth as 12-on-11) [1], and another suggesting

an 11-on-10 structure [2]. None of these studies, however,

had the necessary spatial sensitivity to unambiguously resolve

this inconsistency. Only surface X-ray diffraction (SXRD) has

the necessary resolution (approximately two parts in ten-

thousand of an in-plane reciprocal lattice unit), and hence

SXRD studies were performed on g/Ru at the Materials Science

beamline of the SLS.

Surprisingly, in-plane SXRD measurements showed that the

moiré structure agrees with neither of those previously pro-

posed, but is in fact unambiguously 25-on-23, having a pe-

Figure 1: (a) The vertical displacement field (in Å) of the corruga-

ted supercell of graphene on Ru, which consists of four, not

quite identical, subunits; (b) The ruthenium substrate is also

slightly corrugated, in antiphase to the graphene.

18 Research focus and highlights – Synchrotron light PSI Scientific Report 2008

riodicity of over 60 Å [3]. This superstructure comprises four

translationally inequivalent (but nonetheless nearly identical)

subunits [see Figure 1(a)] with chemistries very similar to that

of the initially proposed 12-on-11 structure.

Out-of-plane measurements along superstructure rods showed

pronounced oscillations and indicated both strong out-of-

plane corrugation of the graphene with an amplitude of 1.4Å,

and also a weaker corrugation of the Ru. More recent analysis

of the data using a parametric approach implemented in GenX,

which uses a genetic algorithm [4], shows that the corrugation

of the Ru is 180o out of phase with that of the graphene

[Figure 1(b) and [5]].

Dipole rings in the h-BN nanomesh

h-BN nanomeshes on Rh(111) and on Ru(0001) were also

studied using SXRD and showed registries of 13-on-12 [6] and

14-on-13, respectively [7]. Strong modulations of the super-

structure rods also indicate significant modulations of the

h-BN and substrate. This corresponds well to STM studies of

h-BN on Rh, where a clear corrugation of the surface was

observed [8].

In contrast to graphene, the h-BN nanomesh is not a metal [9]

and a difference in the electronic and electrostatic landscape

between the regions close to the substrate (holes) and those

further away (wires) is expected. These differences can be

measured by angle-resolved photoemission-spectroscopy

(ARPES). The difference in electronic structure between the

holes and wires is reflected in a splitting of the σ bands [Figure 2(a)], but because of the absence of any states at the Fermi

level this has no immediate effect on the lateral electrical

resistance. However, this splitting reflects the different elec-

trostatic potentials in the holes and on the wires. This differ-

ence in the local work function can also be probed through

the adsorption of a closed shell species such as xenon, as is

visible from the different core-level lines for adsorbed Xe in

the holes and on the wires [H and W in Figure 2(a)].

The difference of 300 meV in electrostatic energy at the Xe

atom sites indicates a lateral local electrostatic field on the

rims of the holes. This dipole field locally enhances the bond-

ing of atoms or molecules that may be polarized. In order to

test this hypothesis, we performed thermal-desorption spec-

troscopy measurements on adsorbed Xe. Detailed analysis of

the respective Xe core-level intensities on the holes and wires

as a function of temperature [Figure 2(a)] indicates that the

Xe bond energy on the holes and the wires is almost the same,

except for the last 12 Xe atoms in every hole. These Xe atoms

form a ring at the rim of the holes, where the dipole field is

strongest, and are trapped there up to significantly higher

temperatures [10].

These results indicate that every hole of the nanomesh has a

dipole ring which significantly enhances its trapping potential.

This is further illustrated by the ability to trap Cu-phthalocy-

anine (Cu-Pc) molecules at room temperature, as shown in

Figure 2(b). As on most other substrates, the molecules can

move within the holes, resulting in the diffuse shapes. How-

ever, they cannot cross the dipole ring once they are trapped.

Similar trapping mechanisms are expected for all molecules

and atoms, where the maximum trapping temperature de-

pends on their size and polarizability.

The h-BN nanomesh is robust in air and even water, thus with

the regular spacing of the dipole rings and the relatively easy

preparation of large-scale samples the technological relevance

of more than 1011 molecular traps per square mm is self-evi-

dent.

References

[1] S. Marchini et al., Phys. Rev. B 76 075429 (2007).

[2] A. L. Vázquez et al., Phys. Rev. Lett. 100 056807 (2008).

[3] D. Martoccia et al., Phys. Rev. Lett. 101 126102 (2008).

[4] M. Björck, G. Andersson, J. Appl. Cryst. 40 1174 (2007).

[5] D. Martoccia et al., unpublished.

[6] O. Bunk et al., Surf. Sci. 601 L7 (2007).

[7] D. Martoccia et al., unpublished.

[8] M. Corso et al., Science 303 217 (2004).

[9] T. Brugger et al., Phys. Rev. B 79 045407 (2009).

[10] H. Dil et al., Science 319 1824 (2008).

Figure 2: (a) Three-dimensional rendered photoemission data set

of the desorption of Xe from the h-BN nanomesh on Rh(111);

(b) STM image of Cu-Pc molecules trapped in the holes of the

nanomesh at room temperature.

PSI Scientific Report 2008 Research focus and highlights – Synchrotron light 19

X-ray absorption spectroscopy (XAS) has long been estab-

lished as a precise method of measuring local structure in

disordered systems such as molecular systems in solution.

This technique has recently been introduced into the domain

of ultrafast science where the electronic and nuclear dynam-

ics of molecules and crystals are examined on the time scales

of atomic motion [1, 2]. In the present investigation, ultrafast

XAS has been used to examine the photocatalytic excited state

of the [Pt2(P2O5H2)4]4– (PtPOP) anion (see Figure 1) dissolved

in ethanol.

Time-resolved X-ray absorption spectroscopy

An X-ray absorption spectrum is obtained by measuring either

the transmission or total fluorescence of a sample as a func-

tion of incident X-ray photon energy. A typical measurement

allows the reconstruction of atomic distances on the scale of

Measurements were performed at the MicroXAS beamline at

the Swiss Light Source by exciting a 10 mM PtPOP solution in

ethanol with 100 fs laser pulses at 390 nm and probing at the

Pt L3 absorption edge (11.56 keV). The transient XAS spectrum

(excited minus unexcited), shown in Figure 2a, directly reflects

the electronic and structural changes that occur 150 ns after

excitation. In this study, the EXAFS region of the XAS spectrum

has been exploited to determine the excited-state structure

of PtPOP.

Retrieving excited-state structures

The ability to retrieve photoinduced structural changes with

high accuracy is based on a rigorous model-based fitting ap-

proach. By including prior knowledge in the form of physi-

cally reasonable distortion models, the number of free fitting

parameters can be reduced considerably, allowing the intro-

duction of additional parameters, such as the photoexcited

population and the energy shift between excited and ground-

state XAS spectra, which are typical for time-resolved XAS

analyses and often difficult to obtain by independent methods.

The general procedure followed is to first obtain accurate

structural values for the ground state of the system, then to

use these values as a starting point for the excited-state

structure. By making physically reasonable changes to the

ground state structure according to a specific distortion

model, then simulating the EXAFS spectrum for the new

structures, the resulting transient EXAFS spectra can be cal-

culated by subtracting the ground-state fit. For each excited-

state structure, the difference between the experimental and

simulated transient spectra can be minimized by introducing

fitting parameters such as the energy shift and the photoex-

cited population. This procedure can then be repeated with

various realistic structural distortion models that all involve

a contraction along the Pt-Pt axis, allowing the result to con-

verge to the smallest difference between experiment and

calculation.

In this way, the best fit was obtained for a Pt-Pt contraction

of 0.31(6) Å and a Pt-ligand elongation of 0.013(5) Å (see

Figure 2) [5]. The latter is larger than just resulting from the

Pt-Pt contraction, which indicates that the coordination bonds

are weakened upon the Pt-Pt bond formation in the excited

state. This small Pt-P elongation has been predicted by DFT

calculations [4], but this represents the first experimental

confirmation of such a structural change and illustrates the

sensitivity of both time-resolved XAS as a technique to resolve

excited-state structures and the analysis procedure used.

Remarkably, the bridging P-O-P ligands do not follow the Pt

atoms in the contraction movement, which supports the

weakening of the Pt-P bonds and the rigidity of these bidentate

ligands. In addition, the analysis indicates an excitation

population of 7% and a zero energy shift. Both of these con-

clusions seem accurate: optical measurements indicate an

excited-state contribution of approximately 8%, and no en-

ergy shift of the excited-state X-ray absorption spectrum is

expected as the photoexcitation does not affect the charge

density on the Pt atoms.

It should be emphasized that the present transient EXAFS

analysis goes beyond the simple determination of nearest-

neighbour distances. By using a model-based fitting approach,

a more global picture of the excited molecule can be obtained.

Application of this analysis technique to other photocatalytic

systems should provide a wealth of information not directly

available through other methods.

References

[1] C. Bressler et al., Chem. Rev. 104 1781 (2004).

[2] C. Bressler et al., Science 323 489 (2009).

[3] R. M. van der Veen et al., CHIMIA 62 287 (2008).

[4] I.V. Novozhilova et al., J. Am. Chem. Soc. 125 1079

(2003).

[5] R. M. van der Veen et al., Angew. Chem. Int. Ed.

48 2711 (2009).

Figure 2: a) Static Pt L3 XAS spectrum of PtPOP in solution (black

line, left axis) and the transient (excited – unexcited) XAS

spectrum (red circles, right axis, same units as left) integrated up

to 150 ns after excitation; b) Transient EXAFS data (circles)

and best fit (solid line, see text). The best-fit structural distor-

tions are indicated in the upper right corner.


Transfer of information is a basic property of biological sys-

tems, with common examples including the transfer of

genetic information or nerve impulses. The transmission of

signals occurs at an even more fundamental level and is

mediated by signaling molecules, which bear a phosphate or

a sulfate group. Since these processes are of supreme impor-

tance, they have been extensively studied and a number of

mechanisms and related protein structures have been

revealed. ASST is unusual amongst sulfuryl transfer enzymes

in that it exhibits a previously unknown three-dimensional

structure. This novel topography was revealed by X-ray crystal-

lography at the SLS [1].

The crystal structure of ASST, at 2 Ångström resolution, re-

vealed that ASST contains an extremely unusual disulfide

bond. In ASST, this bond is characterised by an extremely

short distance between the two linked cysteine residues and

a high steric strain, which we believe can only be efficiently

formed by the action of the disulfide bond formation machin-

ery genetically associated with ASST [2]. This disulfide bridge

is a prerequisite for proper folding of this protein and could

also play a role in regulating its catalytic activity. More striking

than this unusual disulfide bond geometry, however, was the

overall structure of ASST. This consisted of two equal propel-

ler-like parts which contain active sites in the central funnel

formed by the beta-sheet ‘blades’ of each of the propellers.

Such a fold has never before been observed for a sulfotrans-

ferase, leading to fundamental questions regarding the struc-

ture-function relationship of ASST.

In order to answer these questions, two complementary

approaches were adopted: we replaced individual amino

acids and probed the biophysical properties of these mutant

forms of ASST, while concomitantly treating the native form

of ASST with molecules acting as sulfuryl-donors and solving

the crystal structure of these native intermediates. Mutations

of ASST showed five nitrogen-containing amino-acids to be

essential for function.

These residues build a reaction cage which accommodates

both the donor and the acceptor of the sulfuryl group. Fur-

thermore, during sulfotransfer, the sulfuryl group is directly

(covalently) bound to a histidine side chain of ASST. Thus, the

signal is first transferred from the donor to ASST and subse-

quently from ASST to the acceptor. Such a ping-pong mecha-

nism is unique in the processes of sulfuryl transfer.

As a number of histidine residues surround the active site of

ASST, in order to clarify the catalytic role of each residue,

Together with researchers from ETH Zurich, we have shed light on the protein aryl-sulfate sulfotransferase

(ASST), present in pathogenic E. coli bacteria, which cause urinary tract infections. In addition to an entirely

new structure, we uncovered a transfer mechanism similar to ping-pong, whereby the “ball” is kept in a previ-

ously unknown way.

Robin L. Owen, Clemens Schulze-Briese, Swiss Light Source, PSI; in collaboration with Goran Malojc̆ić,

John P. A. Grimshaw, Maurice S. Brozzo, Hiang Dreher-Teo and Rudi Glockshuber, Institute of Molecular Biology

and Biophysics, ETH Zurich

Structural and biochemical basis for novel sulfuryl transfer mechanism

Figure 1: Ribbon diagram highlighting the β-propeller fold of ASST. The six blades of the propeller are individually coloured

while the small N-terminal β-sandwich domain is yellow.


electrospray ionization mass spectrometery was performed

on both the native and sulfurylated forms of the enzyme.

Together with the crystal structure of native ASST, results from

these experiments clearly demonstrated that His-436 is the

residue that undergoes transient covalent sulfurylation during

catalysis. Structural analysis of the two intermediate forms of

ASST showed, for the first time, this high-energy sulfuryl-

histidine intermediate state, confirming the proposed ping-

pong reaction pathway.

The experiments summarised here provide a basis for under-

standing sulfuryl transfer in a manner independent of the

universal sulfuryl donor (adenosine 3’-phosphate-5’-phos-

phosulfate, PAPS) in mammals, opening up medically interest-

ing perspectives. ASST is a promising target for antibacterial

drugs, and together the crystal structures and biochemical

data provide a basis for drug design targeting this virulence

factor.

It is also interesting to note that these insights were only made

possible by combining crystallographic, spectroscopic [3], and

other biochemical methods. An advanced form of mass spec-

trometry, combined with multiple crystallographic models

enabled us to understand the architecture of the active site

and thus elucidate the catalytic pathway of the enzyme.

The complete account of the work described here can be found

in reference [1].

References

[1] G. Malojc̆ić, R. L. Owen, J. P. A. Grimshaw, M. S. Brozzo,

H. Dreher-Teo and R. Glockshuber, PNAS, 105

19217–19222 (2008).

[2] J. P. A. Grimshaw et al., J Mol Biol, 380 667–680 (2008).

[3] R. L. Owen et al., J Synchrotron Rad, 16 173–182 (2009).

Figure 2: Close view of the central funnel of native ASST showing a sulfuryl group in the active site. A full understanding of the active

site was only possible after combining multiple crystallographic and biochemical experiments.


During lung development, the airways and an extensive gas

exchange area have to be formed. The development usually

starts with the appearance of two lung buds. At the terminal

ends of the buds, a repetitive process starts where elongation

of the future airways alternates with branching. After approx.

20 rounds of outgrowth and branching, the ducts and parts

of the respiratory airways are formed. During alveolarization,

the gas exchange area is further enlarged by a subdivision of

the terminal air spaces by the formation of new septa. One

leaflet of the double-layered capillary network inside the exist-

ing septa folds up and gives rise to a new double-layered

capillary network within the newly forming septa (Figure 1,

A–C). Later, during microvascular maturation, the double-

layered capillary network of the alveolar septa is reduced to

a single-layered one (Figure 1D). Currently, it is believed that

after this phase the lifting off of new septa from preexisting

ones is excluded due to the missing second capillary layer.

Consequently, after microvascular maturation is completed,

the enlargement of the gas exchange area will be achieved by

lung growth and not by addition of new alveolar septa. By the

same token, a mature alveolar septum, once lost, will most

likely not be reformed. Therefore, a noteworthy amount of

lung regeneration is excluded, according to this view. The time

when alveolarization in humans stops is not well-defined and

has been discussed for decades. Currently, many agree on an

age of 2–3 yr [1] whereas older data suggested that the forma-

tion of new alveoli ceases at ca. 8 yr or even at 16–18 yr of

age [2]. Nevertheless, one question remained open: how may

new alveoli be formed at a later time point? It has been pro-

posed that (i) late alveolarization may take place in subpleu-

ral areas where a double-layered capillary network is not re-

quired or (ii) late alveolarization may follow a different,

unknown mechanism. So far, alveolarization after the phase

of microvascular maturation is on debate, and the question

on how any form of “late” alveolarization may take place re-

mains open.

The large clinical relevance of late alveolarization inspired us

to follow two directions. First, we applied a stereological

method by estimating the length density of the alveolar en-

trance rings and developed a novel approach to follow the

formation of new alveolar septa throughout lung development

and growth. Second, we were wondering how the requirement

We have been challenging the historical view of lung development which states that the formation of new

alveolar septa from preexisting ones ceases due to the reduction of a double- to a single-layered capillary

network inside the alveolar septa. Synchrotron-based tomographic microscopy investigations of developing rat

lungs have shown that new alveolar septa are forming until young adulthood – mainly by lifting off from mature

septa containing single-layered capillary networks. This newly discovered second phase of lung alveolarization

imposes new precautions when using drugs influencing structural lung development.

Marco Stampanoni, Swiss Light Source (SLS), PSI and Institute for Biomedical Engineering, ETH Zurich and

University of Zurich; Sonja Mund, Johannes Schittny, Institute of Anatomy, University of Bern

Unfolding the lung: understanding the alveolarization process

Figure 1: Formation of new septa during classical alveolarization

(A-C) and microvascular maturation (D).


of a double-layered capillary network inside the existing

alveolar septa may be overcome. For this purpose, we studied

3D tomographic data sets of vascular casts of rat lungs ob-

tained at the TOMCAT beamline of the SLS.

Figure 2 shows the lumen of the capillaries. Inside the cavity

of an alveolus, the up-folding of the single-layered capillary

network is observed (blue dashed lines in A, C, and E). The

folding is indicative of the formation of a new septum. The 3-D

visualization enabled us, for the first time, to look at the re-

verse side of the same septum (B, D, and F). At the basis of

the folding, we detected a local duplication of the existing

capillary network (covering of the blue dashed line in B, D,

and F). Whereas most duplications are already formed in these

examples (arrowhead), one is most likely just forming by

sprouting angiogenesis (arrow in B). In addition, (forming)

tissue posts inside the capillary network (holes in the vascu-

lar cast, green asterisk) are indicative for intussusceptive

angiogenesis (the growth of the capillary network to allow the

up-folding).

We were able to show that the requirement of a double-layered

capillary network at the site of septation is still valid; how-

ever, the two layers do not have to be preexisting as cur-

rently postulated, but they may be formed rapidly and locally

by angiogenesis when needed. Because microvascular matu-

ration takes place during alveolarization, we defined the entire

time when new septa/alveoli are formed during lung develop-

ment and growth as “developmental alveolarization”. This

term distinguishes the developmental processes from any

kind of lung regeneration, which we called “regenerative al-

veolarization”.

Synchrotron-radiation tomographic microscopy was essential

for the structural understanding on how new alveoli are formed

throughout lung development and growth. We could show

that new alveoli are formed not only before, but also after, the

maturation of the alveolar microvasculature. During the latter,

the requirement of a double-layered capillary network at the

site where a new septum will be formed is overcome by a local

duplication found at the sides of septation. Most likely, many

of these duplications were not preexisting. We defined the

classically described alveolarization “phase one of develop-

mental alveolarization” and the newly described form “phase

two”. Until now, the understanding of phase two is based on

structural evidence only. However, due to its clinical signifi-

cance, we believe that these structural findings will be the

starting point for investigations of the molecular mechanisms

involved. The description of phase two will most likely force

us to rethink our views of (i) lung regeneration and of (ii) side

effects on the structure of the lungs during the treatment of

children and adolescents with glucocorticoids and retinoids.

References

[1] J. H. Caduff, L. C. Fischer and P. H. Burri, Scanning

electron microscopic study of the developing

microvasculature in the postnatal rat lung, Anat Rec 216

154–164 (1986).

[2] E. R. Weibel, Morphometry of the Human Lung

(Springer-Verlag, Heidelberg, 1963).

[3] J. C. Schittny, S. I. Mund and M. Stampanoni, Evidence

and structural mechanism for late lung alveolarization,

Am J Physiol Lung Cell Mol Physiol 294 L246–L254

(2008).

Figure 2: 3D visualizations of the capillary network of single

alveoli. The entrances of the alveoli are labeled with a yellow

dotted line. Mercox vascular casts of 21-day-old rat lungs

were imaged at 12.6 keV with a pixel size of 0.7 microns. Scale

bars are 10 microns. See text for details.


Synchrotron radiation X-Ray powder diffraction (SR-XRPD)

experiments require detection systems with low noise, high

dynamic range and high angular (FWHM) and d-spacing reso-

lution. These requirements can only be fulfilled by single-

photon counting systems with high granularity [1]. The

MYTHEN detector (Microstrip sYstem for Time rEsolved ex-

perimeNts) has been designed to fulfil all these demands and,

furthermore, to perform time-resolved measurements. High-

resolution powder diffraction patterns acquiring 120° in 2θ can be collected in a fraction of a second.

Detector description

The MYTHEN detector consists of more than 30,000 independ-

ent channels (µstrips) working in parallel and positioned at

760 mm from the centre of the diffractometer, with a pitch of

50 µm. This results in an intrinsic detector angular resolution

of 0.004° [2].

The detector is based on a silicon micro-strip sensor absorb-

ing the diffracted X-rays and coupled to a custom-made inte-

grated circuit [3].

Thanks to its single-photon counting capability, the detector

is virtually noiseless and has a dynamic range of up to 24 bits.

The fluctuation in the number of detected photons is purely

Poisson-like, and thus the data quality is maximized, with low

statistics. The low noise of the front-end electronics allows

the detection of photons of energy down to 5 keV, while the

short shaping time of the analogue signal permits counting

rates of up to 1 MHz/channel. The channels are read out in

parallel, with an inter-frame dead time of 0.3 ms. The maximum

frame rate of the whole detector is limited by the data transfer

rate and is about 10 Hz for the whole detector (increasing to

300 Hz for a 5° partial readout and 16 bits dynamic range).

Acquisition times down to 100 ns are possible and can be

synchronized to users’ experiments using external signals. A

small on-board memory can store 4 to 32 frames in real time,

depending on the dynamic range. Data acquisition with

MYTHEN is possible through a user-friendly graphical interface

and is completely integrated in the beamline control system.

An upgraded version of MYTHEN was installed at the SLS

powder diffraction station in July 2007 and has been available

for users since the beginning of 2008, providing excellent

data quality.

Applications

Some examples of experiments showing the outstanding

performance of the MYTHEN detector are:

1) Bragg crystallography

MYTHEN has worked remarkably well, not only for time-re-

solved applications but also for structural solution and refine-

ment. Here, time resolution is usually not relevant and, there-

fore, the intensity of the incoming photon beam is generally

sacrificed to achieve an optically aberration-fre

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